Improving avocado orchard productivity through disease management
This article appears in the Winter 2019 edition of Talking Avocados (Volume 30 No 2).
By Liz Dann, Akila Prabhakaran, Emily Lancaster, Kaylene Bransgrove, Montana Hickey & Eugenie Singh
The disease management project AV16007 started in November 2018 and concludes in May 2022. There are several activities being undertaken, across the major root, fruit and nursery diseases impacting avocado productivity in Australia. This article presents an overview of the project and update on experimental results obtained so far. More detailed research articles will be published in future issues of Talking Avocados.
Monitoring phosphonate residues in fruit
This activity is being conducted in collaboration with Graeme Thomas, GLT Horticulture, and several growers and agronomists across Australia who have kindly sent fruit samples for analyses. Hard green fruit was collected at commercial maturity from 40 blocks across Queensland and Western Australia. The ranges of phosphorous acid residues are presented in Table 1.
|Region||Number of blocks||Minimum phos. acid (mg/kg fresh)||Maximum phos. acid (mg/kg fresh)|
|Central and southeast QLD||13||3||93|
The maximum residue limit (MRL) for fruit sold within Australia is 500mg/kg, so none of the fruit tested came close to exceeding domestic MRL. Growers and exporters should be mindful of MRLs imposed by our current and potential overseas markets of all crop protectants used within the orchard, (see the updated MRL comparison table at in the Best Practice Resource Library, under the Export heading (www.avocado.org.au/bpr/). Results from the phosphonate residue testing have been communicated to participating growers and the Avocado Export Project Reference Group, and we are currently collecting Bundaberg/Childers fruit from the same blocks for the second year of testing. Information provided by growers on dates of phosphonate applications and concentrations in roots will be analysed with fruit residue data, providing important information on optimal delivery and timing of applications. We will also look for correlations with crop load (yield) and tree age.
Biofumigation for Phytophthora infested replant sites
A trial was initiated in northern NSW at a site where trees declining from Phytophthora root rot had recently been removed. Despite the high natural inoculum, we broadcast wheat grain colonised with Phytophthora cinnamomi to ensure consistent and even distribution of inoculum. A row (about 100m long) was cultivated and some plots sown with either Caliente 199TM or BQ Mulch, which are both Brassica cover crops commercialised specifically for their biofumigation effects, or left fallow. At flowering, the brassica crops were mown with several passes of the slasher and incorporated by rotary hoe. Chicken manure was incorporated into half of the fallow plots, and the other half left as untreated controls. One side of the row was covered in heavy duty black builders film, as shown in (Figure 1) for two weeks. Nursery trees (Hass on Reed) were planted two weeks after removal of the black plastic, with eight trees per treatment plot (four in each of the plastic covered and uncovered halves of the plots). Tree health and other growth parameters has been assessed at least monthly since planting. At six months after planting, the health of trees in covered plots was significantly better than those in uncovered plots (Figure 2), although there were no statistical effects amongst biofumigation treatments. We will continue to record tree health for the next couple of months.
Field trials to assess effects of new chemicals and soil amendments on tree health, fruit yields and quality
This is a major component of the project. Two trials in south-west Western Australia and one trial in Central Queensland were established nearly 12 months ago in partnership with growers and agronomists. Treatments common to each site include woodchip + chicken manure + gypsum (as the recommended “best practice”), as well as Mineral Mulch (building board waste with high available Si and Ca, see Talking Avocados, Summer 2018) and anti-oomycete metalaxyl-M + unregistered product. There are additional microbial formulations, biochar and other mulches specific to each trial. Tree health assessments have been undertaken at each visit. Our first fruit harvest is coming up in July for the Childers trial, with WA later in the year. We will be assessing yields and packouts for each treatment, as well as levels of postharvest disease, anthracnose and stem end rot. We will sample leaves, fruit and soil for major nutrients and look for useful correlations as potential predictors of tree health and fruit quality. The treatment programs and assessments at all sites will continue until the conclusion of the project in May 2022.
Stem end rot, graft dieback (nursery), branch cankers and branch dieback
A range of fungi are associated with fruit stem end rot, graft dieback (in the nursery and sometimes after planting), branch cankers and branch dieback. These include Botryosphaeriaceae family (eg. Lasiodiplodia theobromae and Neofusicoccum parvum), Colletotrichum spp., Pestalotiopsis sp. and Diaporthe sp. While we frequently isolate these fungi onto agar media, we have little knowledge about which are the most pathogenic, ie. cause the most severe disease, or how they enter the fruit or branch. A PhD student commenced in April 2019 to investigate the fungi associated with the different symptoms in the major growing regions of Australia. So far more than 200 isolates have been collected from several orchards and nurseries, and these will be accurately identified by microscopy and molecular DNA sequencing. Any patterns showing which fungi are more prevalent with a particular symptom type or region will be determined. Modes of infection will be investigated, eg, whether they are carried as symptomless infections in nursery trees (as “endophytes” ), or enter via wounds or at flowering. A field trial in an unsprayed orchard in SE QLD is planned for later this year, where several treatments including fungicides and microbial/biological products will be sprayed at flowering and resulting fruit collected and assessed for development of stem end rot. This trial will also provide some information about new fungicides or products which may be safe to apply without burning sensitive flowers.
Some preliminary lab and glasshouse work by an undergraduate student has shown that L. theobromae and N. parvum causing Stem End Rot and disease in fruit (Figure 3) are able to colonise the seed coat and seed (Figure 4). When these infected seed are planted (Figure 4b) the seedling stems are also infected by these fungi even though they may not show obvious symptoms. This can potentially cause graft failure in the nursery (Figure 5), or dieback from the graft after planting, as the infection may remain dormant in the graft until trees are stressed. It is extremely important that nurseries collect only clean fruit and extract the seed when the fruit is still hard prior to ripening (but must be physiologically mature). This removes the seed coat and reduces the risk of transfer of these endophytic fungal pathogens. This recommendation has been included in the recently-revised guidelines for ANVAS nurseries, and is available in the Nursery Industry Accreditation Scheme Australia (NIASA) Best Management Practice Guidelines, 7th Edition, updated 2018, Appendix 13 Avocado High Health Production. The guidelines are an excellent resource for any nursery, and can be purchased for $99 from http://nurseryproductionfms.com.au/niasa-accreditation/).
Other diseases and activities
The project also includes field and glasshouse experiments evaluating Trichoderma and fungicide drench management options for Phellinus brown root rot and black root rot (see Talking Avocados Spring 2017). There is also a significant industry support component including participation in grower field days and biosecurity advisory panels. We also interact directly with growers, agronomists and nursery operators and process samples to assist with diagnosing diseases or non-disease disorders, which may be impacting tree health or fruit quality. One of the more challenging of these has been the tree lodging problem (see Talking Avocados, Summer 2019 edition for more on that), which most likely arose from planting root-bound trees, rather than from a disease.
The Improving avocado orchard productivity through disease management (AV16007) project has been funded by Hort Innovation, using the avocado research and development levy and contributions from the Australian Government.
- Dann and project collaborators (2018) Does silicon amendment benefit avocado tree health or fruit quality? Talking Avocados 28(4): 53-56.
- Parkinson, E. Dann and R. Shivas (2017) Black root rot of avocado – what do we know and how can we manage it? Talking Avocados 28(3):35-39.
- Dann, K. Bransgrove, S. Newett, G. Thomas and several growers (2019) Lodging of avocado trees, Talking Avocados 29(4):40-45.
 Endophytic fungi internally infect living plant tissues without causing any visible disease for at least part of their life cycle
This article was prepared for the Winter 2019 Talking Avocados magazine.
Date Published: 30/08/2019